Improve performance with duplicate check on paste

[Siedlerchr]

Run duplicate check on background thread

In 90% cases better performance, except if you have plenty of duplicates (UI dialog is shown then)

Fixes https://github.com/JabRef/jabref-issue-melting-pot/issues/574

Side effect, UI might freeze if you copy a large number of entries because of respawning the duplicate entry dialog. Not happy about it,

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[LoayGhreeb]

Does this also cover import entries from web searches and "Import into current library" in the File menu?

[Siedlerchr]

Yes, should be at least for the Web search, that uses the ImportHandler, for the import into current file I am not sure

[github-actions]

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